The present invention relates to cast alloys having high strength and plasticity, and more specially, to cast high-carbon alloys.
The cementite Fe3C produces effective hardening of steels both being part of pearlite and as a separate phase in high-carbon compositions. Cast high-carbon steel includes the cementite phase in a form of rough platelets and layers along the grain boundaries. Such steel is hard but brittle. Certain high temperature treatments can cause cementite to coagulate to provide cementite having a spherical structure referred to as grain pearlite or grain cementite.
Annealing processes can produce steel having grain cementite structure. Hard and brittle high-carbon steel having a carbon content greater than about 0.8% can be transformed into the plastic state by heat treatment. Annealing high-carbon steel at high temperature without air access for three days caused cementite platelets to coagulate into a grain cementite structure. Anosov, P. P., O bulatach, Gorny Journal, kn. II:157 (1841). However, excessive annealing temperature results in carbon separation and precipitation (i.e., graphitization). More contemporary processing guides recommend either long annealing at a temperature just below the austenite-ferrite Al phase transition or an annealing process with the temperature oscillation temperature near the Al phase transition. Long, high-temperature annealing produces large cementite particles characterized as particle diameter greater than about 10 xcexcm. The strength of such steels is low.
More than two millenniums ago, craftsmen of India and Persia discovered a method for developing Damascus steel, a high-carbon steel that is extremely strong, non-brittle, and having a characteristic surface pattern. Russian metallurgist P. P. Anosov studied Damascus steel and, through systematic investigation, developed a process for the production of Russian bulat (Damascus) steel at the Zlatoust plant (Ural). Anosov""s research determined that Damascus steel is high-carbon steel that has been subjected to an intensive forging at high temperature. Microscopic investigation was used for the first time in this research for analysis of technological process in steels. Anosov noted that in the highest quality forged bulats xe2x80x9cbroken lines become shorter and transform into pointsxe2x80x9d. The formation of small cementite spherical particles was later confirmed later in detailed microstructural investigations of Damascus steel. See, e.g., Belaiew, N. T., Damascene Steel, Journal of Iron and Steel Institute, Vol. 47:417 (1921); Sherby, O. D., Ultrahigh-carbon Steels, Journal of Metals, 50-55 (June 1985); and Verhoeven, J. D., Damascus Steel, Part III: The Wadsworth-Sherby Mechanism, Materials Characterization, Vol. 24:205-227 (1990). The microstructure of a blade made from Damascus steel is illustrated in FIG. 5. The microstructure includes cementite particles having diameters from about 4 to about 20 xcexcm, which were produced during forging from original thick layers of cementite. The cutting edge of a Damascus sword is a saw-like structure with sharpened particles supported in a plastic matrix. Small, submicron-size cementite particles between these rows were produced during the thermomechanical treatment from the original thin interlayers of cementite in pearlite. These submicron cementite particles are responsible for high strength of Damascus steel, which is an example of a composite material having small hard particles supported in a soft plastic matrix. The analysis of diffusion effects across the decarburization zone in a typical Damascus sword permitted to estimate the forging temperature in the range 740-760xc2x0 C. and a lower limit of forging time of around 4 hour. See, Verhoeven 1990.
Fundamental scientific research on the development of high-carbon steel with the uniform submicron-size cementite structure was performed by Sherby in the 1970s and 1980s. The Sherby method for forming high-carbon steel closely resembles the method of Damascus craftsmen. Both methods include numerous cycles of intensive high temperature deformation and annealing. While Damascus steel is formed by forging, the Sherby method utilizes rolling as the deformation means.
U.S. Pat. No. 3,951,697, issued to Sherby, describes an ultra high-carbon steel having a carbon in excess of about 1.0% and an iron grain matrix with uniformly dispersed cementite. The iron grain in the steel being stabilized in a predominantly equiaxed configuration having an average grain size no greater than about 10 microns, and the cementite being in predominantly spheroidized form in a temperature range of 723xc2x0 C. to 900xc2x0 C. Sherby""s method for forming such steel includes the steps of heat treating at least 500xc2x0 C. and mechanically working the heat-treated steel under sufficient strain deformation to form an iron grain matrix with uniformly dispersed cementite.
The Sherby patent describes a representative thermal mechanical process for developing the high-carbon steel with extremely small size both of iron grains and cementite particles:
A casting of the 1.3%C steel was heated to 1130xc2x0 C. for 60 minutes and then was rolled continuously, in fifteen passes, at 15% per pass, to a true strain to 2.0. Since the original casting cooled during rolling it experienced deformation in gamma range as well as gamma plus cementite range. When a temperature of 565xc2x0 C. was reached it was rolled isothermally in this ferrite plus cementite range to an additional true strain of 0.8 (again, at 10% per pass). The microstructure of the warm worked steel reveals a fine spheroidized structure with ferrite grains in the order of one micron and less. The room temperature properties of the material were as follows: (1)the Rockwell xe2x80x9cCxe2x80x9d hardness of the plate was 46, and (2) tensile tests revealed a yield strength of 195 ksi, an ultimate tensile strength of 215 ksi and tensile elongation of 4.2%. The high temperature properties reveal this material to be superplastic with 480% elongation to fracture at 650xc2x0 C. when deformed at a strain rate one percent per minute.
Damascus steel generally has the structure described by Sherby. The relationship between the Sherby method and the method for producing the Damascus steel has been described. See, e.g., Wadsworth. (1980) and Sherby (1985).
Several subsequent patents related to further improvements of the superplastic propeties of high-carbon steels by means of the optimization of their composition and regimes of the thermal mechanical processing. These patents include U.S. Pat. No. 4,448,613, entitled xe2x80x9cDivorced eutectoid transformation process and product of ultrahigh-carbon steelsxe2x80x9d, which describes additional thermal treatments, with and without deformation of high-carbon steels with Cr, Mn and Si for improving the structure; U.S. Pat. No. 4,533,390, entitled xe2x80x9cUltra high-carbon steel alloy and processing thereofxe2x80x9d, which describes increasing by means of higher concentration of Si and Cr the eutectoid temperature so that superplastic processing may proceed at high strain rates and low stress levels at elevated temperatures; and U.S. Pat. No. 5,445,685, entitled xe2x80x9cTransformation process for production of ultrahigh-carbon steels and new alloysxe2x80x9d, which describes increasing the temperature for superplastic deformation by means of higher concentration of Al, Cr and Mn and cooling the steels from temperature of dissolving the major part of carbides (Al+50xc2x0 C.) with the controlled cooling rate to obtain a steel having substantially spheroidized cementite.
Despite the advancements made in developing processes for forming high-carbon steel, deficiencies in these processes remain. Important negative effects of the long high-temperature heat treatment in all above mentioned methods include the tendency of the grain size to grow and the coagulation of carbides. These effects may decrease catastrophically the strength of materials. Although, the best mechanical characteristics of high-carbon steels may be achieved when these negative effects are overcome at least partially by means of rather high concentration of carbide-forming additives and the demand to perform the intensive high temperature deformation in a narrow temperature window, both of these approaches increase the cost of materials. Accordingly, there exists a need for methods for forming high-carbon steel having high strength and plasticity. The present invention seeks to fulfill this need and provide further related advantages.
In one aspect the present invention provides cast high-carbon steel having high strength and plasticity and a method for its formation. In accordance with the method, castings from high-carbon steels are formed by a casting process through temperature control that provides the formation of metal in high-strength and plastic condition. The microstructure of the high-carbon steel formed in accordance with the invention is characterized as having a small grain size and microspherical form of carbide particles dispersed substantially throughout the alloy matrix. Articles formed by the method possess high strength and high plasticity. The high-carbon steel formed by the method has a structure similar to the structure of Damascus steel. However, unlike Damascus steel, which is formed by repetitive forging deformation (e.g., hammering), the method of the invention does not include cycles of high temperature deformation and annealing.
In the method of the invention, forming during cooling from the melting point through temperature control results in a structure without rough cementite plates that are characteristically formed by conventional processes. Instead, the method provides small size cementite grains supported in a plastic matrix, a structure analogous to Damascus steel.
In another aspect, the invention provides alloys other than high-carbon steel produced by the process of this invention. Alloys may include other matrix elements, for example, nickel, titanium, zirconium, aluminum, among others. The composition of an alloy and optimal temperature control of the cooling from the liquid state can be selected for producing a strengthening phase in the form of small size spherical particles. Principal additives in alloys can produce carbides, borides, nitrides, oxides and/or intermetallides of appropriate size and in appropriate quantity.